Development of Glasses Transmitting Bactericidal Radiation
JSGT 1950 V34 T153-T172
The development is described of “soft” glasses with good transmission to 2537 Å and suitable for the construction of efficient sources of bactericidal radiation (“germicidal” lamps). A satisfactory glass has the percentage composition SiO2 65, B2O3 2, Na2O 5·5, K2O 9·5, BaO 18 and contains about 0·01% iron oxide expressed as Fe2O3 the iron being reduced to the ferrous state by addition of aluminium powder to the batch. Sulphates present as impurities in the raw materials (for example, in the barium carbonate) or deliberately added for refining purposes reduced the ultraviolet transmission of the glass; and small amounts of titania, present as an impurity in many sands, were also harmful. A soda–lime–silica glass of percentage composition SiO2 75, CaO 10, Na2O 15 also transmitted well at 2537 Å even when it contained about 0·01% iron oxide, in contrast to the earlier findings of D. Starkie & W. E. S. Turner, who failed to obtain transmission at such short wavelengths in a glass of the same composition. Changes of basicity or of soda–potash ratio within the limits studied had a negligible effect on ultraviolet transmission. Melts in the series Na2O–B2O3 showed that the transmission to 2537 Å decreased with increase in Na2O content, but the transmission of any particular glass depended markedly on the type of raw materials used in its preparation. Melts made using borax were superior to those using sodium carbonate as the source of Na2O, and the addition of a small percentage of aluminium powder to the batch also improved the transmission. Lithium borate glasses were slightly better and potassium borate glasses a little worse than corresponding sodium borate glasses; but nothing absolute is claimed for these results, owing to unavoidable impurities even in the purest available raw materials. It was shown that small percentages of iodine in a sodium borate glass caused marked absorption of 2537 Å radiation. Sodium borate glasses containing a small percentage of sodium sulphate, melted under reducing conditions, may be colourless, blue or yellow, depending upon the Na2O content, the ultraviolet transmission being reduced in all cases by the presence of sulphur. Inclusion of 0·5% ZnO or CdO in a sodium borate suppressed the sulphur blue colour and increased the ultraviolet transmission; a similar inclusion of CaO, MgO or BeO failed to suppress the blue colour, although the addition of BeO improved the 2537 Å transmission.
J. E. Stanworth
